The production and removal of thermal electrons has been studied in ${\mathrm{O}}_2$ and airlike ${\mathrm{N}}_2$: ${\mathrm{O}}_2$ mixtures between 1 and 10 Torr during continuous and intermittent irradiation by 1.5-MeV electrons. The use of a large reaction chamber and a spatially uniform electron beam flux reduces diffusion losses two orders of magnitude below those in previous experiments, and permits direct measurements of electron attachment at correspondingly lower total gas pressures. The observed rate coefficients for three-body attachment of thermal electrons at 300°K are $k_3\left({\mathrm{O}}_2\right)=(2.12\mathrm{}\pm 0.14)\times {10}^{-30\mathrm{}}$ ${\mathrm{cm}}^6$/sec and $k_3(4\mathrm{}{\mathrm{N}}_2: {\mathrm{O}}_2)=(1.10\mathrm{}\pm 0.07)\times {10}^{-31\mathrm{}}$ ${\mathrm{cm}}^6$/sec, both during irradiation and in the afterglow. The effective cross sections for ionization by 1.5-MeV electrons and all resulting secondaries are ${\sigma }_e\left({\mathrm{O}}_2\right)=(2.83\mathrm{}\pm 0.7)\times {10}^{-18\mathrm{}}$ ${\mathrm{cm}}^2$ and ${\sigma }_e(4\mathrm{}{\mathrm{N}}_2: {\mathrm{O}}_2)=(2.45\mathrm{}\pm 0.7)\times {10}^{-18\mathrm{}}$ ${\mathrm{cm}}^2$. These effective cross sections are larger than recently measured cross sections for primary ionization by a factor of 3.0 ± 0.8 in both gases.

• Received 1 August 1968

DOI:https://doi.org/10.1103/PhysRev.178.175